The increased demand for data communication and the remarkable growth of the internet have resulted in increased demand for communication capability within metropolitan areas. There has also been an equally large increase in demand for communication capability between large metropolitan areas. Optical communication systems using a network of fiber optic cables are being developed and installed to meet the increased demand.
The data transmission capacity of fiber optic cables and fiber optic networks has been substantially increased as a result of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). Within WDM and DWDM systems, optical signals assigned to respective wavelengths are combined (multiplexed) into a multiple wavelength signal for transmission over a single fiber optic cable or other suitable waveguide. A typical DWDM system modulates multiple data streams on to different portions of the light spectrum. For example, one data stream may have an assigned wavelength of 1543 nanometers (nm) and the next data stream may have an assigned wavelength of 1543.8 nm. The required spacing between assigned wavelengths is generally established by International Telecommunications Union (ITU) specifications. These spacings include 0.4 nm and 0.8 nm.
Wavelength division demultiplexing (WDDM), the reverse process of multiplexing, typically refers to separation of a multiple wavelength signal transmitted by a single optical fiber or other suitable waveguide into constituent optical signals for each wavelength. Each optical signal may be further processed to obtain the associated data stream or other information. Both multiplexing and demultiplexing are required for satisfactory operation of WDM and DWDM systems. Multiplexing and demultiplexing of optical signals in conventional DWDM systems are typically performed by two separate optical devices which are relatively expensive and often difficult to manufacture.
Typical grating based spectrum analyzers and wavelength division multiplexers and demultiplexers that use optical fibers or other types of waveguides have passbands or spectral responses that are generally highly peaked with a slow roll off in their wavelength response. This characteristic results from diffraction response of the associated grating element that separates the wavelengths and transmission response of intervening optical lens elements and receiving optics. Such responses (peaked with slow roll off) do not use the full bandwidth of most multiplexers and demultiplexers. As a result it is often difficult to specify wavelength tolerances for associated components such as laser light sources, amplifiers and other optical components.
Various techniques may be used to transform or defocus multiple wavelength optical signals to achieve some spectral broadening. Previously available techniques and procedures generally create significant losses due to spreading of the multiple wavelength signals in directions which are both parallel and perpendicular to the direction of dispersion. Therefore, transforming or defocusing multiple wavelength optical signals has generally been avoided for most optical communication systems.
For conventional multiplexers and demultiplexers, the width of each passband profile for associated optical signals generally corresponds with the core diameter of respective input and output optical elements. If the diameter of a core is approximately equal to ten micrometers (10 μm), the passband profile of an associated optical signal would often be ten micrometers or less in spatial domain which is relatively small. Sometimes, the thickness of the associated cladding layers may be reduced to increase the core-to-cladding diameter ratio at the input or output and thus the associated passbands profile.